Method and apparatus for locating patterns in an optical image

ABSTRACT

The invention provides methods and apparatus for processing an image to identify the position of a linear pattern--for example, a line or a cross-hair comprising a plurality of intersecting lines. The system performs a first processing step for generating a projection of the image along axes aligned with an expected position of the linear patterns. A second processing step performs a mirror symmetry filtering on the projection to bring out a single peak corresponding to the center of the linear pattern. To further isolate that peak, the system performs a further filtering operation to remove peaks of lesser slope angle, so that only a highly sloped spike corresponding to the linear pattern will remain. The position of the center that peak corresponds to the center of the linear pattern in the original input signal.

This application is a continuation of U.S. patent application Ser. No.07/828,241 filed on Jan. 30, 1992, now abandoned, for "Method andApparatus for Locating Patterns in an Optical-Image", the contents ofwhich are hereby expressly incorporated.

FIELD OF THE INVENTION

This invention relates to machine vision, and more particularly, tomethods and apparatus for accurately locating linear patterns, e.g., across-hairs, in an optical image.

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. The copyright owner has noobjection to the facsimile reproduction by anyone of the patent documentor the patent disclosure, as it appears in the Patent and TrademarkOffice patent file or records, but otherwise reserves all copyrightrights whatsoever.

BACKGROUND

Automated product assembly and manufacturing processes often rely onmachine vision to determine the position of a component being processed.Typically, a linear pattern, such as a cross-hair, is used as areference point for precision alignment.

Various methods of machine vision positioning are currently practiced.Among the primary ones are variations on the general Hough transform andcorrelation (template matching). With respect to the latter, thereexists binary exclusive-OR correlation and, more recently, gray-scalenormalized correlation. One common element of these prior art methods isthat they require, as input, an image template of the pattern they areto locate.

As a component undergoes various stages of assembly or manufacture, theappearance of the alignment pattern may change. For example, etching andmasking of a semiconductor wafer can alter the contrast, definition andthickness of cross-hairs embedded in the wafer. Thus, in order to beeffective during the entire assembly process, the prior art methodstypically require many image templates. Unfortunately, the actualappearance of a locating pattern on a component is not alwayspredictable, frustrating even the use of multiple ideal template images.

A further drawback of prior art systems is that they consume excessivetime in comparing actual optical images to the multiple template images.

In view of the foregoing, an object of the invention is to provide animproved vision system and, more particularly, improved methods andapparatus for accurately locating the center of a linear pattern, e.g.,a line or a cross-hair, in an optical image.

Still another object of the invention is to provide a machine visionpositioning system that does not rely on an object template in order toposition a piece.

Yet another object of the invention is to provide a system capable ofpositioning a component notwithstanding changes in its appearance duringprocessing.

SUMMARY OF THE INVENTION

The aforementioned objects are attained by the invention, which providesmachine vision methods and apparatus capable of quickly and accuratelylocating linear patterns, such as lines, or cross-hairs made up ofintersecting lines, on a component.

In one aspect, the invention provides an apparatus for processing acomponent image to identify the position of a linear pattern thereon.The apparatus includes a first processing element for generating aprojection of the image along an axis substantially aligned with anexpected orientation of the linear pattern. As described further below,a projection is essentially a one-dimensional "profile" of the inputimage.

If a linear pattern in the image is aligned with the projection axis,edges of that pattern will correspond with peaks on an otherwise flat,or substantially flat, profile. An apparatus according to the inventionfurther includes a processing element for performing "mirror symmetry"filtering on the projection. That filtering relies on the symmetry ofthe edge peaks to bring out a single peak corresponding to the center ofthe linear pattern.

To isolate that center peak, the apparatus includes a notch detectionelement that operates on the mirror symmetry filter output to filter outlesser peaks of low slope angle, so that only a highly sloped spikecorresponding to the linear pattern of interest will remain.

The center of that peak corresponds to the center of the linear patternin the original input signal. The apparatus, accordingly, includes apeak finding element that determines the location of the peak center. Toimprove accuracy of that determination, the apparatus can interpolate apeak position from the apparent peak and the two neighboring points onopposite sides of that apparent peak.

An apparatus of the type described above can determine the location of aline in an input image, e.g., a "street" in a semiconductor wafer.Alternatively, it can to locate a single "hair" of a cross-hair pattern.

The invention accordingly provides, in another aspect, an apparatus forlocating the center of a cross-hair, made up of intersecting lines,contained in an input image. That apparatus is constructed and operatedas described above, and is additionally adapted to take projectionsalong two axes, each of which is aligned with an expected orientation ofthe corresponding hairs. That apparatus also performs mirror symmetryand notch-detection filtering on both projections to permit location ofthe centers of each hair. The center of the cross-hair is, then,determined to lie at the intersection of the hairs.

In still other aspects, the invention provides methods for linearpattern location corresponding to the operation of the apparatusdescribed above.

As will be appreciated from this summary, and from the subsequentdetailed description, features of the invention include its ability tolocate linear patterns without matching them with image templates, asotherwise required in the prior art. Another feature of the invention isits ability to detect very low contrast linear patterns and patternedges. This results, in part, from the use of projections which greatlyenhance the signal-to-noise ratio of the image signal. Moreover, theinvention demands two-dimensional processing only for the generation ofprojections, utilizing single-dimensional processing for all othersophisticated operations.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features, objects and benefits of the invention can be moreclearly understood with reference to the following description of anillustrative embodiment, and to the drawings, in which:

FIG. 1 is a schematic representation of a preferred embodiment of anapparatus according to the invention;

FIG. 2 is a perspective view of a linear pattern on a surface;

FIG. 3 is a planar view of a representative segment of an image signaland corresponding projections;

FIG. 4a is a plot of a projection signal;

FIG. 4b is a detail of the plot of a projection signal taken along linesA--A;

FIG. 4c is a plot of a mirror symmetry signal;

FIG. 4d is a plot of an output of a notch-detection element; and

FIG. 5 is a diagram showing the function of the peak locating element.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT

With reference now to FIGS. 1 and 2, a schematic representation of anapparatus 10 for accurately locating a cross-hair 12, comprisingintersecting linear patterns 12a, 12b is shown on a surface 14.

The surface 14 is illuminated, for example, by lamps 16 to facilitatethe creation of an image signal by a video camera 18. The output of thatcamera is electrically connected to an optional storage device 20 thatcan store the image prior to processing. As shown in the illustration,the output of storage device 20 is routed to a projection element 30,which performs the first stage of location processing, as discussedbelow.

Video camera 18, lamps 16, and storage device 20 operate and areconfigured in a manner conventional in the art.

A digital image signal 22 output by camera 18 is made up of individualpixels, I_(x),y. A representative portion 23 of an image signal 22 isillustrated in FIG. 3, showing pixels designated I₁,1, I₁,2, . . . I₄,5.

The projection element 30 sums pixels along directions parallel to axes36a, 36b corresponding to the expected orientation of the linearpatterns 12a, 12b to be inspected. For example, if the linear patternsare oriented with the pixel grid itself, the projection summations areconducted along the grid's rows and columns; otherwise, they areconducted in appropriate directions. The projection element generates,for those sums, projection signals 32a, 32b, respectively.

The aforementioned projection signals can be generated in a mannerconventional in the art. Preferably, they are generated in accord withthe techniques revealed in U.S. Pat. No. 4,972,359, assigned to theassignee hereof. A software listing of a more preferred implementationof projection element 30 is provided in Appendix A, filed herewith.

By way of example, illustrated projection signal 32a includes elementsPa₁ through Pa₄, generated in accord with the following mathematicalfunction (which assumes that the pixel grid is aligned with the axes36a, 36b). ##EQU1## Projection signal 32b, including elements Pb1through Pb5, is generated by a like formula.

As noted above, each projection signal 32 provides a profile of theimage signal 22 in a direction parallel to the corresponding axis. Thus,the linear pattern aligned with the axis is emphasized, while image"noise" is effectively averaged out.

For sake of simplicity, the discussion which follows focuses onprocessing of a projection signal corresponding to a single linearpattern, e.g., "hair" 12a of cross-hair 12. It will be appreciated that,in order to determine the location of cross-hair 12, like processing isperformed on the other "hair" 12b.

An exemplary projection signal 32 is illustrated in FIG. 4a, where threefeatures of interest are shown: the portions 36, corresponding to theaverage background intensity value of the image scene; the portion 39corresponding to the average intensity value of the central region ofthe linear pattern or "hair"; and the portion 38, corresponding to theaverage intensity of the boundary between the linear feature and thebackground. It will be appreciated that the features 38 and 39 can berelatively brighter than, darker than, or equal to, the background 36,in any combination. It will be further appreciated, however, thatfeatures 38 and 39 cannot both be equal to the background, because thepattern would not be detectable. Further fluctuation in the illustratedprojection signal 32 is due, for example, to noise in the image signal22.

The image processor 10 performs further processing on the projectionsignal in order to provide an accurate indication of the location of thecenter of the linear pattern 12a on the surface 14. Particularly,referring to FIG. 1, exemplary projection signal 32 is processed bymirror symmetry filter element 50, using an input radius r, to derive apeak corresponding to the center of the pattern 12a. The radius, r,represents one-half of the expected width of the linear pattern, and isillustrated as 42 of FIG. 4b.

The radius r is input, for example, by the user, e.g., via console 90.The value of radius r is selected to accommodate the expected variationof thickness in the linear pattern caused by, for example, processing ofthe underlying component. While the exact value of the radius, r, is notcritical, it is preferably selected to be slightly larger than thelargest expected half-width of the linear pattern.

Mirror symmetry filter element generates the filtered signal, S, fromexemplary projection signal P and corresponding radius, r, in accordwith the mathematical function ##EQU2## for each value of i between rand the length of the projection signal P minus r.

Alternatively, the mirror symmetry filter element can generate thefiltered signal, S, in accord with the mathematical function ##EQU3##

Those skilled in the art will appreciate that Equations 2a and 2b, whilesimilar, produce resultant signals S which are effectively one-halfpixel out of phase with one another.

With respect to Eqs. 2a and 2b, at each point i along an axis 36, theprojection signal 32 value between zero and r on the left side of thepoint is subtracted from the value the same distance to the right of thepoint. An absolute value is calculated, and is added to the absolutevalues of the differences at the other points between zero and r. Thisgives an indication of how mirror symmetric the projection 32 is aboutpoint i. The same algorithm is run along the length of the projectionsignal 32, giving an indication of the overall symmetrical quality ofthe projection signal 32.

As illustrated in FIG. 4c, the mirror symmetry signal 52 is low,indicating a high degree of symmetry, in regions of approximatelyuniform background, e.g., at distances more than one radius away fromthe linear pattern 12a. Within one radius of the pattern, the value of Srises, indicating a low degree of symmetry. The signal S remainsrelatively high at all points within radius r of the center of thepattern, except for a very small region surrounding the exact center ofthe feature. This is revealed as a very sharp, downward spike 56 insignal S, whose extreme point 54 corresponds to the center of theexemplary linear pattern 12a.

The apparatus 10 performs still further processing in order to identifythe location of the center of the linear pattern 12a. The goal is todistinguish the low signal value 56, corresponding to the true center ofthe linear pattern 12a, from the other low values corresponding to theuniform background. This is performed by notch detection element 60,which operates on the mirror symmetry signal S with a mask to generate apeak location signal L further emphasizing the peak 56.

The notch detector element 60 generates the peak location signal L inaccord with the mathematical function: ##EQU4## based on a mask, M,which can be depicted as follows: ##STR1##

In a preferred embodiment, the notch mask has a central notch one pixelwide of negative one unit amplitude. On either side of the notch, thereare one pixel wide shoulders of zero amplitude. At the far ends, theshoulders notches are two pixels wide and positive one-half unit high.That is, p=2, z=1 and n=1, while P=1/2, Z=0 and N=-1. This preferredmask is as follows:

    1/2 1/2 0 -1 0 1/2 1/2

In effect, a mask of this shape represents a negative-going notchlocator, filtering out all areas of the symmetry signal 52 which are lowdue to the uniform background. The result of the operation of thisfilter on the mirror symmetry signal 52 of FIG. 4c is illustrated inFIG. 4d.

The spike 62 and its peak 64 represent an accurate indication of therelative position of the center 11a of the exemplary linear pattern 12aparallel the axis 36 of interest.

In a less preferred embodiment, the function of notch detection element60 may be performed by a laplacian filter element, which convolves themirror symmetry signal S with a mask to generate a peak location signalL further emphasizing the peak 56.

As those skilled in the art will appreciate, convolution involves takingthe "dot product" of successive overlapping windows of the mirrorsymmetry signal S with a pixel mask M, having elements M₁, M₂, . . .M_(k). Thus, the laplacian estimator element 60 generates the peaklocation signal L in accord with the mathematical function: ##EQU5##

A mask for such an operation is as follows:

    1/4 -1/4 0 1 0 -1/4 -1/4

A further refinement of the position of the peak 56 is made possible byimplementing a peak location element 70 as illustrated in FIG. 1.

Referring to FIG. 5, the point which has the highest amplitude in spike62--i.e., the point representing the apparent peak of the peak locationsignal L--is identified as point 72. On either side of that point 72 areneighboring points 74 and 76.

In order to better determine the actual peak, and thereby the actualcenter of the linear pattern 12a, the peak location element 70determines the location of a point 84 representing the intersection oflines 78 and 80. The line 78 is determined as that line which connectsapparent peak 72 and lowest-value neighbor 76, while line 80 isdetermined as that line passing through highest-value neighbor 74 andhaving a slope 82 which is the negative of that of line 78, asillustrated.

The point 84 where the two line segments 78, 80 intersect is identifiedas a truer approximation of the center 11a of the linear pattern 12a.

In an alternative to the aforementioned method of peak detection, theprocessor 10 generates two mirror symmetry signals, S₁ and S₂, from eachprojection signal, P. The first mirror symmetry signal S₁ is generatedaccording to Eq. 2a, while the second, S₂, is generated according to Eq.2b. Each of the signals S₁, S₂ are then passed through notch detectionelement 60 to produce notch detection signals L₁ and L₂, respectively.Peak detection proceeds as described above, except insofar as apparentpeak point 72 is determined by signal L₁, while neighboring points 74and 76 are determined by signal L₂.

In order to determine the coordinates 11 of a cross-hair pattern 12, theapparatus 10 generates a projection signal Pa, Pb of the image signal Ialong respective axes 36a, 36b. It then uses the input radius rassociated with each of those projections. The apparatus, in turn,determines a mirror symmetry signal Sa, Sb corresponding to respectiveones of the projections signals Pa, Pb. And, from those mirror symmetrysignals Sa, Sb, the apparatus generates peak location signals La, Lb,respectively.

Upon locating the center of the corresponding linear patterns 12a, 12bin the manner described above, the apparatus 10 generates a signalrepresenting the center of the cross-hair 12 at the point ofintersection of the two patterns 12a, 12b. That signal may be then beoutput to terminal 90 or positioning apparatus, not shown.

A further understanding of the invention may be attained by reference toAppendix A, filed herewith, disclosing a preferred softwareimplementation of aspects of the invention described and claimed herein.

SUMMARY

Described above are improved machine vision methods and apparatuscapable of quickly and accurately locating linear patterns, such aswafer streets or cross-hairs, on a component. Unlike the prior art,systems constructed in accord with the invention does not require aninput template pattern.

Those skilled in the art will appreciate that the embodiments describedabove are illustrative only, and that other systems in the spirit of theteachings herein fall within the scope of the invention. Thus, forexample, it will be appreciated that methods and apparatus other thanthat described above for mirror symmetry filtering can be used.Likewise, the invention embraces methods other than notch detection(e.g., the Laplacian technique disclosed above) for filtering the outputof the mirror symmetry filter. Still further, the use of peak detectionmethodologies and apparatus other than, but falling within the spiritof, those described above is contemplated by the invention. Moreover, itwill be appreciated that the methods and apparatus described herein canbe used to identify linear patterns other than "streets" andcross-hairs.

And, of course, that systems constructed in accord with the inventionhave applicability outside the field of semiconductor wafer processing.Thus, for example, they can be used to locate cross-hairs in connectionwith graphics printing of color overlays, and in positioning componentsduring parts assembly or inspection.

These and other such uses, as well as modifications, additions anddeletions to the techniques described herein may fall within the scopeof the invention. ##SPC1##

I claim:
 1. An apparatus for identifying a linear pattern, having anexpected radius less than or equal to a value r, in an image signal, I,said apparatus comprising:A. projection means for generating aprojection signal, P, representative of a projection of said imagesignal along an axis substantially aligned with an expected orientationof said linear pattern, where said projection signal, P, is comprised ofelements P_(i), where i is an integer between 1 and a length of saidimage signal along said axis, B. mirror symmetry filter means, coupledwith said projection means, responsive to said radius and saidprojection signal for filtering that projection signal to generate amirror symmetry signal, S, representative of the degree of symmetry ofthe projection signal P about each point therein, wherein said mirrorsymmetry filter means includes means for filtering said projectionsignal P to generate two mirror symmetry signals, S₁ and S₂, whereineach signal S₁, S₂ is representative of the degree of symmetry of theprojection signal P about each point therein, and wherein each signalS₁, S₂ is out of phase with the other, C. notch detection means, coupledwith said mirror symmetry filter means, for operating on said mirrorsymmetry signal with a selected mask to generate a peak location signalemphasizing a peak in said mirror symmetry signal corresponding to alocation of a center of said linear pattern, and D. peak locating means,coupled with said notch detection means, for identifying a peak in saidpeak location signal, and for estimating from a location of that peak alocation of a center of said linear pattern.
 2. An apparatus foridentifying a cross-hair comprising a plurality of intersecting linearpatterns, having expected radii less than or equal to a value r, in animage signal, I, said apparatus comprising:A. projection means for agenerating, for each of said linear patterns, a corresponding projectionsignal, P, representative of a projection of said image signal along anaxis substantially aligned with an expected orientation of that linearpattern, where said projection signal, P, is comprised of elementsP_(i), where i is an integer between 1 and a length of said image signalalong the respective axis, B. mirror symmetry filter means, coupled withsaid projection means, and responsive to said radius and each saidprojection signal, for filtering that projection signal to generate acorresponding mirror symmetry signal, S, representative of the degree ofsymmetry of the projection signal P about each point therein, whereinsaid mirror symmetry filter means includes means for filtering saidprojection signal P to generate two mirror symmetry signals, S₁ and S₂,wherein each signal S₁, S₂ is representative of the degree of symmetryof the projection signal P about each point therein, and wherein eachsignal S₁, S₂ is out of phase with the other, C. notch detection means,coupled with said mirror symmetry filter means, for operating on eachsaid mirror symmetry signal with a selected mask to generate acorresponding peak location signal emphasizing a peak in each saidmirror symmetry signal corresponding to a location of a center of thecorresponding linear pattern, D. peak locating means, coupled with saidnotch detection means, for identifying a peak in each said peak locationsignal, and for estimating from a location of that peak a location of acenter of the corresponding linear pattern, and E. center point locatingmeans, coupled with said peak locating means, for determining the centerof said cross-hair pattern to lie at an intersection of the estimatedlocations of said linear patterns.
 3. An apparatus according to any ofclaim 1 and 2, wherein said mirror symmetry filter means includesA.means for filtering a projection signal to generate said mirror symmetrysignal, S₁, comprising elements S₁ i, wherein each element S₁ i has avalue determined in accord with the mathematical function ##EQU6## B.means for filtering a projection signal to generate said mirror symmetrysignal, S₂, comprising elements S₂ i, wherein each element S₂ i has avalue determined in accord with the mathematical function ##EQU7##
 4. Anapparatus according to claim 3, whereinA. said notch detection meansincludes means for operating on each said mirror symmetry signal, S₁,S₂, to generate corresponding peak location signals, L₁, L₂, B. saidpeak locating means includesi) means for identifying in said peaklocation signal L₁ a position and amplitude of a most significant peak,ii) means for identifying positions and amplitudes of a neighboringpoint on each side of a most significant peak in said peak locationsignal L₂, and iii) means for estimating from the positions andamplitudes of those neighboring points a location of the center of saidlinear pattern.
 5. An apparatus for identifying a linear pattern, havingan expected radius less than or equal to a value r, in an image signal,I, said apparatus comprising:A. projection means for generating aprojection signal, P, representative of a projection of said imagesignal along an axis substantially aligned with an expected orientationof said linear pattern, where said projection signal, P, is comprised ofelements P_(i), where i is an integer between 1 and a length of saidimage signal along said axis, B. mirror symmetry filter means, coupledwith said projection means, responsive to said radius and saidprojection signal for filtering that projection signal to generate amirror symmetry signal, S, representative of the degree of symmetry ofthe projection signal P about each point therein, C. notch detectionmeans, coupled with said mirror symmetry filter means, for operating onsaid mirror symmetry signal with a selected mask to generate a peaklocation signal emphasizing a peak in said mirror symmetry signalcorresponding to a location of a center of said linear pattern, whereinsaid notch detection means includes means for operating on said mirrorsymmetry signal S with a selected mask in accord with the mathematicalfunction ##EQU8## where N, P and Z are elements of said mask, each ofrespective lengths n, p and z, and D. peak locating means, coupled withsaid notch detection means, for identifying a peak in said peak locationsignal, and for estimating from a location of that peak a location of acenter of said linear pattern.
 6. An apparatus for identifying across-hair comprising a plurality of intersecting linear patterns,having expected radii less than or equal to a value r, in an imagesignal, I, said apparatus comprising:A. projection means for agenerating, for each of said linear patterns, a corresponding projectionsignal, P, representative of a projection of said image signal along anaxis substantially aligned with an expected orientation of that linearpattern, where said projection signal, P, is comprised of elementsP_(i), where i is an integer between 1 and a length of said image signalalong the respective axis, B. mirror symmetry filter means, coupled withsaid projection means, and responsive to said radius and each saidprojection signal, for filtering that projection signal to generate acorresponding mirror symmetry signal, S, representative of the degree ofsymmetry of the projection signal P about each point therein, C. notchdetection means, coupled with said mirror symmetry filter means, foroperating on each said mirror symmetry signal with a selected mask togenerate a corresponding peak location signal emphasizing a peak in eachsaid mirror symmetry signal corresponding to a location of a center ofthe corresponding linear pattern, wherein said notch detection meansincludes means for operating on said mirror symmetry signal S with aselected mask in accord with the mathematical function ##EQU9## where N,P and Z are elements of said mask, each of respective lengths n, p andz, and D. peak locating means, coupled with said notch detection means,for identifying a peak in each said peak location signal, and forestimating from a location of that peak a location of a center of thecorresponding linear pattern, and E. center point locating means,coupled with said peak locating means, for determining the center ofsaid cross-hair pattern to lie at an intersection of the estimatedlocations of said linear patterns.
 7. An apparatus according to any ofclaims 5 and 6, wherein said notch detection means includes means forgenerating said peak location signal L by operating on said mirrorsymmetry signal with a mask signal, M, said mask signal having anamplitude that peaks toward the center.
 8. An apparatus according toclaim 7, wherein said notch detection means includes means forgenerating said peak location signal L by operating on said mirrorsymmetry signal with a mask signal, M, said mask signal having elementsM_(k), where k is between 1 and 7, and where those elements haverespective values 1/2, 1/2, 0, -1, 0, 1/2, 1/2.
 9. A method foridentifying a linear pattern having an expected radius less than orequal to a value r, in an image signal, I, said method comprising:A. aprojection step for generating a projection signal, P, representative ofa projection of said image signal along an axis substantially alignedwith an expected orientation of said linear pattern, where saidprojection signal, P, is comprised of elements P_(i), where i is aninteger between 1 and a length of said image signal along said axis, B.a mirror symmetry filter step responsive to said radius and saidprojection signal for filtering that projection signal to generate amirror symmetry signal, S, representative of the degree of symmetry ofthe projection signal P about each point therein, wherein said mirrorsymmetry filter step includes a step for filtering said projectionsignal P to generate two mirror symmetry signals, S₁ and S₂, whereineach signal S₁, S₂ is representative of the degree of symmetry of theprojection signal P about each point therein, and wherein each signalS₁, S₂ is out of phase with the other, C. a notch detection step foroperating on said mirror symmetry signal with a selected mask togenerate a peak location signal emphasizing a peak in said mirrorsymmetry signal corresponding to a location of a center of said linearpattern, and D. a peak locating step for identifying a peak in said peaklocation signal, and for estimating from a location of that peak alocation of a center of said linear pattern.
 10. A method foridentifying a cross-hair comprising a plurality of intersecting linearpatterns, having expected radii less than or equal to a value r, in animage signal, I, said method comprising:A. a projection step for agenerating, for each of said linear patterns, a corresponding projectionsignal, P, representative of a projection of said image signal along anaxis substantially aligned with an expected orientation of that linearpattern, where said projection signal, P, is comprised of elementsP_(i), where i is an integer between 1 and a length of said image signalalong the respective axis, B. a mirror symmetry filter step forresponding to said radius and each said projection signal for filteringthat projection signal to generate a corresponding mirror symmetrysignal, S, representative of the degree of symmetry of the projectionsignal P about each point therein, wherein said mirror symmetry filterstep includes a step for filtering said projection signal P to generatetwo mirror symmetry signals, S₁ and S₂, wherein each signal S₁, S₂ isrepresentative of the degree of symmetry of the projection signal Pabout each point therein, and wherein each signal S₁, S₂ is out of phasewith the other, C. a notch detection step for operating on each saidmirror symmetry signal with a selected mask to generate a correspondingpeak location signal emphasizing a peak in each said mirror symmetrysignal corresponding to a location of a center of the correspondinglinear pattern, D. a peak locating step for identifying a peak in eachsaid peak location signal, and for estimating from a location of thatpeak a location of a center of the corresponding linear pattern, and E.a center point locating step for determining the center of saidcross-hair pattern to lie at an intersection of the estimated locationsof said linear patterns.
 11. A method according to any of claims 9 and10, wherein said mirror symmetry filter step includesA. a step forfiltering a projection signal to generate said mirror symmetry signal,S₁, comprising elements S₁ i, wherein each element S₁ i has a valuedetermined in accord with the mathematical function ##EQU10## B. a stepfor filtering a projection signal to generate said mirror symmetrysignal, S₂, comprising elements S₂ i, wherein each element S₁ i has avalue determined in accord with the mathematical function ##EQU11## 12.A method according to claim 11, whereinA. said notch detection stepincludes a step for operating on each said mirror symmetry signal, S₁,S₂, to generate corresponding peak location signals, L₁, L₂, B. saidpeak locating step includesi) a step for identifying in said peaklocation signal L₁ a position and amplitude of a most significant peak,ii) means for identifying positions and amplitudes of a neighboringpoints on each side of a most significant peak in said peak locationsignal L₂, and iii) means for estimating, from the positions andamplitudes of those neighboring points a location of the center of saidlinear pattern.
 13. A method for identifying a linear pattern, having anexpected radius less than or equal to a value r, in an image signal, I,said method comprising:A. a projection step for generating a projectionsignal, P, representative of a projection of said image signal along anaxis substantially aligned with an expected orientation of said linearpattern, where said projection signal, P, is comprised of elementsP_(i), wherein i is an integer between 1 and a length of said imagesignal along said axis, B. a mirror symmetry filter step responsive tosaid radius and said projection signal for filtering that projectionsignal to generate a mirror symmetry signal, S, representative of thedegree of symmetry of the projection signal P about each point therein,C. a notch detection step for operating on said mirror symmetry signalwith a selected mask to generate a peak location signal emphasizing apeak in said mirror symmetry signal corresponding to a location of acenter of said linear pattern, wherein said notch detection meansincludes means for operating on said mirror symmetry signal S with aselected mask in accord with the mathematical function ##EQU12## whereN, P and Z are elements of said mask, each of respective lengths n, pand z, and D. a peak locating step for identifying a peak in said peaklocation signal, and for estimating from a location of that peak alocation of a center of said linear pattern.
 14. A method foridentifying a cross-hair comprising a plurality of intersecting linearpatterns, having expected radii less than or equal to a value r, in animage signal, I, said method comprising:A. a projection step for agenerating, for each of said linear patterns, a corresponding projectionsignal, P, representative of a projection of said image signal along anaxis substantially aligned with an expected orientation of that linearpattern, where said projection signal, P, is comprised of elementsP_(i), where i is an integer between 1 and a length of said image signalalong the respective axis, B. a mirror symmetry filter step forresponding to said radius and each said projection signal for filteringthat projection signal to generate a corresponding mirror symmetrysignal, S, representative of the degree of symmetry of the projectionsignal P about each point therein, C. a notch detection step foroperating on each said mirror symmetry signal with a selected mask togenerate a corresponding peak location signal emphasizing a peak in eachsaid mirror symmetry signal corresponding to a location of a center ofthe corresponding linear pattern, wherein said notch detection meansincludes means for operating on said mirror symmetry signal S with aselected mask in accord with the mathematical function ##EQU13## whereN, P and Z are elements of said mask, each of respective lengths n, pand z, and D. a peak locating step for identifying a peak in each saidpeak location signal, and for estimating from a location of that peak alocation of a center of the corresponding linear pattern, and E. acenter point locating step for determining the center of said cross-hairpattern to lie at an intersection of the estimated locations of saidlinear patterns.
 15. A method according to any of claims 13 and 14,wherein said notch detection step includes a step for generating saidpeak location signal L by operating on said mirror symmetry signal witha mask signal, M, said mask signal having an amplitude that peaks towardthe center.
 16. A method according to claim 15, wherein said notchdetection step includes a step for generating said peak location signalL by operating on said mirror symmetry signal with a mask signal, M,said mask signal having elements M_(k), where k is between 1 and 6, andwhere those elements have respective values 1/2, 1/2, 0, -1, 0, 1/2,1/2.